Currently, various display devices starting from liquid crystal display devices are used for a variety of purposes. In a general display device, one pixel is configured by three sub pixels displaying red, green, and blue that are three primary colors of light, and color display can be enabled by such a pixel.
However, a conventional display device has a problem in that the range of displayable colors (called a “color reproduction range”) is narrow. FIG. 18 illustrates a color reproduction range of a conventional display device that performs display using three primary colors. FIG. 18 is an xy chromaticity diagram of an XYZ chromaticity system, and a triangle having three points corresponding to three primary colors of red, green, and blue as its vertexes represents the color reproduction range. In addition, in the figure, colors of various objects (see Non-Patent Literature 1) existing in the natural world, which are revealed by Pointer, are plotted using x marks. As can be understood from FIG. 18, there are object colors that are not included in the color reproduction range, and a display device performing display using three primary colors is not capable of displaying some of the object colors.
Thus, in order to broaden the color reproduction range of the display device, a technique for increasing the number of primary colors used for display to be four or more has been proposed.
For example, in Patent Literature 1, as illustrated in FIG. 19, a liquid crystal display device 800 in which one pixel is configured by six sub pixels R, G, B, Ye, Cy, and Ma displaying red, green, blue, yellow, cyan, and magenta is disclosed. The color reproduction range of the liquid crystal display device 800 is illustrated in FIG. 20. As illustrated in FIG. 20, the color reproduction range represented by a hexagon having six points corresponding to six primary colors as its vertexes almost covers the object colors. In this way, by increasing the number of primary colors that are used for display, the color reproduction range can be broaden. In this specification, a display device performing display using four or more primary colors will be referred to as a “multi-primary color display device”. In addition, a conventional general display device that performs display using three primary colors will be referred to as a “three-primary color display device”.
As a general format of an image signal that is input to the three-primary color display device, there is an RGB format, a YCC format, or the like. By including three parameters (in other words, a three-dimensional signal) in the image signal (an RGB signal or an YCC signal) of such a format, the luminance levels (gray scale levels) of three primary colors (red, green, and blue) used for display are uniquely determined.
In order to perform display using the multi-primary color display device, it is necessary to convert an image signal of a format used for the three-primary color display device into an image signal including more parameters (four or more parameters). Such an image signal corresponding to the four or more primary colors, in this specification, will be referred to as a “multi-primary color image signal”. In addition, an image signal of a format used for the three-primary color display device, in other words, an image signal corresponding to the three primary colors will be referred to as a “three-primary color image signal” in this specification.